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Butterflies of the Sea

© Esther Wullschleger

Shell-less snails that are living in the sea are mostly extremely colorful - a delight for every diver to see or photograph. These snails of the order Nudibranchiata are called sea slugs, since they haven't got a shell that would protect their soft body from predators or other harm. However, unlike the common name "slugs" might suggest, these marine snails aren't especially closely related to terrestrial slugs. Apart from the fact that the shell is missing in both snail groups, sea slugs haven't got much in common with terrestrial slugs. Sea slugs can afford the "luxury" of not building any protective shell, because they are either well camouflaged on their preferred substrate, while many are also toxic or at least distasteful to potential predators. In the latter case, the colorful appearance serves to warn potential predators, so that these avoid attacking the snails once they have "learned" that they are inedible.

To human observers, the colorful sea slugs are extraordinarily beautiful, especially if they move through the water by flapping their body. This type of locomotion, together with their bright colors, has lent a second common name to the snails: butterflies of the sea. Their scientific name Nudibranchiata, on the other hand, points to the fact that the gills of these snails are placed on the body surface where they float freely in the water, instead of being covered and protected by the snails' tissue. In some species, the gills are reduced to small remnants, in others they are completely missing and replaced by different appendices.

The "butterflies of the sea" are incredibly diverse in ecology and behavior. Among the roughly 5000 to 6000 species that occur in all seas worldwide are such that live only on highly specific substrates, such as corals or plants. These manage to imitate the color and even the structure of their living place amazingly well, so that it might be as good as impossible to spot them there. Furthermore, many species are extremely specialized in nutrition and do only eat very specific plants or animals, such as sea anemones, sponges, algae, bryozoans or fish eggs.

Most sea slugs are around one to five centimetres in size, but some species reach only a few millimetres, while real "giants" of up to ten centimetres length live in the Antarctic and in deep sea. Because many species are rather small, and because their impressive colors show only in live animals under water, sea slugs aren't suitable at all to be shown in exhibitions. Their beauty is rather revealed in photographs - and sometimes does only show under a microscope!

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